In a semiconductor chip containing transistor devices, metal layers function to electrically interconnect the device's different components to one another. Such metal layers generally comprise contacts, which connect the metal layer to the transistor devices and other metal layers, and traces. Aluminum has long been a preferred material for forming the metal interconnect layers, because it is relatively inexpensive and easy to work with.
Unfortunately, aluminum interconnect layers often create problems with the devices formed in a semiconductor chip made of silicon if the device undergoes temperatures in excess of about 400° C. during manufacture of the device. For example, when the aluminum is deposited on a silicon surface the two materials tend to intermix to some degree at their interface. The solubility of silicon in aluminum increases with increasing metallization temperature and approaches about 1 weight percent at about 500° C. as shown by a aluminum silicon phase diagram in FIG. 1. At about 500° C., silicon readily diffuses into aluminum in order to satisfy its solubility requirement. The loss of silicon from the substrate leaves pits behind in the substrate. Likewise, aluminum counter-migrates into the silicon substrate during subsequent deposition or anneal processes. Since very small amounts of aluminum dissolve in silicon, the migrated aluminum fills the silicon depleted regions or pits and forms aluminum filaments. The condition wherein aluminum has migrated into the silicon is known as “spiking.” Spiking can create short circuits in the device when the migrated aluminum spikes through an active region in the silicon device. The aluminum spikes can short a reverse-biased junction and cause a short or excessive leakage of the device.
In order to reduce or prevent aluminum junction spiking problems, the diffusion of silicon into aluminum and vice versa should be minimized. A diffusion barrier layer is typically used to reduce migration of silicon and aluminum into each other. However, providing a diffusion barrier significantly increases the manufacturing costs for micro-electronic devices because multiple steps are often required to pattern and etch the diffusion barrier layer. Since the diffusion barrier layer is usually highly conductive, it must be etched prior to depositing a resistive layer in order to prevent shorting between other devices such as resistors deposited on the semiconductor substrate. There is a need therefore, for improved techniques and processes for reducing spiking caused by diffusion between aluminum and silicon during a semiconductor chip manufacturing process without increasing the number of processing steps required for making such chips.